This invention relates generally to fuel cells and assemblies, more particularly, cells and components thereof as can be configured for use with hydrogen fuel or the direct oxidation of hydrocarbons.
Fuel cells are promising electrical power generation technologies, with key advantages including high efficiency and low pollution. Most potential near-term applications of fuel cells require the use of hydrocarbon fuels such as methane, for which a supply infrastructure is currently available. However, fuel cells typically operate only with hydrogen as the fuel. Thus, current demonstration power plants and planned fuel-cell electric vehicles must include a hydrocarbon fuel reformer to convert the hydrocarbon fuel to hydrogen. Fuel cells that could operate directly on hydrocarbon fuels would eliminate the need for a fuel reformer, providing considerable system and economic advantages and presumably improving the viability of the technology.
Prior art fuel cells utilizing hydrocarbon fuels directly have encountered significant problems. For example, direct-methanol polymer electrolyte fuel cells produce relatively low power densities and require prohibitively large Pt loading of the anodes. In addition, methanol can permeate the electrolyte. See, for instance, Ren, X., Wilson, M. S. and Gottesfeld, S. High performance direct methanol polymer electrolyte fuel cells. J. Electrochem. Soc., 143, L12-L14 (1996); and Wang, J., Wasmus. S. and Savinell, R. F. Evaluation of ethanol, 1-propanol, and 2-propanol in a direct oxidation polymer-electrolyte fuel cell a real-time mass spectrometry study. J. Electrochem. Soc., 142, 4218-4224 (1995). Furthermore, only alcohol fuels appear feasible with this approach.
Alternatively, prior art solid oxide fuel cells (SOFCs) can utilize hydrocarbons directly via internal or external reforming. In this approach, a hydrocarbon fuel (e.g., methane) is combined with H2O and/or CO2, which are typically obtained by recirculating the fuel cell exhaust, and introduced directly to the SOFC anode. Commonly used Ni-based anodes provide the catalyst for the endothermic reforming reactions,
CH4+H2O=3H2+CO xcex94Hxc2x0298=206 kJ/mol CH4xe2x80x83xe2x80x83(1)
CH4+CO2=2H2+2 CO xcex94Hxc2x0298=247 kJ/mol CH4xe2x80x83xe2x80x83(2)
However, maintaining appropriate gas composition and temperature gradients across a large area SOFC stack is challenging. See, Janssen, G. J. M., DeJong, J. P., and Huijsmans, J. P. P. Internal reforming in state-of-the-art SOFCs. 2nd European Solid Oxide Fuel Cell Forum, 163-172, Ed. by Thorstense, B. (Oslo/Norway, 1996); and Hendriksen, P, V., Model study of internal steam reforming in SOFC stacks. Proc. 5th Int. Symp. on Solid Oxide Fuel Cells, 1319-1325, Ed. by U. Stimming, S. C. Singhal, H. Tagawa, and W. Lehnert (Electrochem, Soc., Pennington, 1997).
For instance, if the reforming reactions are slow, then insufficient H2 is supplied to the SOFCs. On the other hand, fast reforming reactions cause cooling localized near the fuel inlet, leading to poor cell performance, and possible cell fracture. Thus, current SOFC stacks of the prior art do not take full advantage of internal reforming; rather, they employ a combination of ≈75% external and 25% internal reforming of hydrocarbon fuels. See, Ray, E. R. Westinghouse Tubular SOFC Technology, 1992 Fuel Cell Seminar, 415-418 (1992).
SOFCs can in principle operate by direct electrochemical oxidation of a hydrocarbon fuel. This approach would be desirable since it eliminates the problems with internal reforming mentioned above, and the theoretical maximum fuel efficiency is as good or better than that for reforming. However, prior art attempts with SOFCs operating at temperatures Tc=900-1000xc2x0 C. with methane fuel have been less than satisfactory: either power densities were very low or carbon deposition was observed. See, Putna, E. S., Stubenrauch, J., Vohs, J. M. and Gorte, R. J. Ceria-based anodes for the direct oxidation of methane in solid oxide fuel calls, Langmuir 11, 4832-4837 (1995); and Aida, T., Abudala, A., Ihara, M., Komiyama, H. and Yamada, K. Direct oxidation of methane on anode of solid oxide fuel cell. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 801-809, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C, (Electrochem. Soc. Pennington, 1995).
Recently, SOFCs have been developed to produce high power densities with hydrogen at reduced temperatures, Tc=600-800xc2x0 C. See, Huebner, W., Anderson, H. U., Reed, D. M., Sehlin, S. R. and Deng, X. Microstructure property relationships of NiZrO2 anodes. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 696-705, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995); dasouza, S., Visco, S J. and DeJonghe, L. C. Thin-film solid oxide fuel cell with high performance at low-temperature. Solid State Ionics 98, 57-61 (1997); Fung, K-Z., Chen, J., Tanner, C. and Virkar, A. V. Low temperature solid oxide fuel cells with dip-coated YSZ electrolytes. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 1018-1027, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995); Minh, N. Q. Development of thin-film solid oxide fuel cells for power generation applications. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 138-145, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995); Godickemeier, M., Sasaki, K. and Gauckler, L. J. Current-voltage characteristics of fuel cells with ceria-based electrolytes. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 1072-1081, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995); Tsai, T. and Barnett, S. A. Increased solid-oxide fuel cell power density using interfacial ceria layers. Solid State Ionics 98, 191-196 (1997); and Tsai, T., Perry, E. and Barnett, S. Low-temperature solid-oxide fuel cells utilizing thin bilayer electrolytes. J. Electrochem. Soc., 144, L130-L132 (1997). However, such systems have not been considered or used for direct-hydrocarbon operation, because carbon deposition reaction rates decrease with decreasing temperature. In fact, there are no known reports SOFC operation on hydrocarbons at Tc less than 800xc2x0 C.
SOFCs and related stacking configurations have undergone considerable development over the past decade. Tubular-cell-based technologies appear to be a promising approach for SOFC stacking. Tubular stacks avoid sealing and manifolding problems inherent to planar stacks, but take a large volume for a given cell active area and can show significant ohmic losses related to current transport around the tube circumference through the (La,Sr)MnO3 (LSM) cathode. Another problem is the relatively poor mechanical toughness of LSM.[N. M. Sammes, R. Ratnaraj, and C. E. Hatchwell, Proceedings of the 4th International Symposium on Solid Oxide Fuel Cells, Ed. By Dokiya, O. Yamamota, H. Tagawa, and S. C. Singhal (Electrochemical Society, Pennington, 1995) p. 952. B. Krogh, M. Brustad, M. Dahle, J. L. Eilertsen, and R. Odegard, Proceedings of the 5th International Symposium on Solid Oxide Fuel Cells, Ed. By U. Stimming, S. C. Singhal, H. Tagawa, and W. Lehnert (Electrochemical Society, Pennington, 1997) p. 1234.] This is typical of SOFC ceramic materials, which are optimized for electrical properties rather than mechanical toughness.
Alternatively, planar stacks can provide higher power-to-volume ratios than tubular stacks, but are not as mechanically robust as tubes and require excellent seals. Another problem with many planar stack designs is that they require pressure contacts between separate SOFC and interconnect plates. This places stringent requirements on the flatness of large-area ceramic plates, making manufacturing difficult and expensive. Furthermore, there are often relatively high resistances associated with these contacts, which deleteriously affect stack performance. It is clear that a choice between tubular and planar stacks involves trade-offs. Even so, the disadvantages associated with each respective approach present obstacles for effective use of SOFCs and suggest a new direction is needed to better utilize and benefit from this technology.
There are a considerable number of problems and the deficiencies associated with the use of hydrocarbons with solid oxide fuel cells. There is a demonstrated need for the use of such fuels in an efficient, economical fashion so as to improve the viability of the related technology.
Accordingly, it is an object of the present invention to provide various solid oxide fuel cells and/or components which can be used with hydrocarbon fuels thereby overcoming various deficiencies and shortcomings of the prior art, including those outlined above. It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all instances, to every aspect of the present invention. As such, the following objects can be used in the alternative with respect to any one aspect of the present invention.
It can be an object of the present invention to increase the rate of hydrocarbon oxidation so as to increase and/or otherwise provide useful power densities. Such densities can be increased and/or provided utilizing various catalytic metals in the fabrication of fuel cell anodes, such anodes as can be used in conjunction with a ceria material.
It can be an object of the present invention to utilize solid oxide fuel cells and/or components thereof for low temperature direct hydrocarbon oxidation.
It can also be an object of the present invention to provide various anodes and related cellular components having small particle size obtainable by sputter deposition processes and/or related fabrication techniques.
It can also be an object of the present invention to provide a method for hydrocarbon oxidation, at a temperature lower than 800xc2x0 C. and/or at a temperature for a specific hydrocarbon whereby there is no carbon deposition.
It can also be an object of the present invention to improve the viability of solid oxide fuel cells, both those described herein as well as those otherwise known in the art, through use of a unique assembly of such cells having a configuration and/or geometry of the type described herein. In particular, it is an object of this invention to provide a cell geometry/configuration whereby all active fuel cell components and interconnects are deposited as thin layers on an electrically insulating support.
It can also be an object of the present invention, in conjunction with one or more of the preceding objectives, to provide a geometry/configuration for an assembly of solid oxide fuel cells whereby assembly design and choice of support material can enhance mechanical durability and thermal shock resistance. A related objective is to decrease overall material cost by providing all cell-active materials in thin layer/film form.
It can also be an object of the present invention to improve a number of solid oxide fuel cell performance or function parameters through integration of the cell components and interconnects on a common support, such advantages including reduction of electrical resistances and interconnect conductivity requirements. As described more fully below, such integration can be accomplished through use of the thin film/layer configurations and related geometries described herein.
Another object of this invention is to provide a cell assembly configuration suitable for SOFCs of the type described herein, especially those operable at low temperatures for direct oxidation of hydrocarbon fuels, such cells as can be prepared to preferentially incorporate the catalytic metal anodes of this invention.
Other objects, features, benefits and advantages of the present invention will be apparent from the following summary and descriptions, and will be readily apparent to those skilled in the art made aware of this invention and having knowledge of various and solid oxide fuel cells in the use of hydrocarbon fuels. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanied examples, tables, data and all reasonable inferences to be drawn therefrom.
The present invention provides for the low-temperature operation of SOFCs using hydrocarbon fuels. High power densities were obtained via direct electrochemical oxidation, without carbon deposition. The results shown herein can be extendable to fuel cell stack operation. In particular and without limitation, the present invention demonstrates the feasibility of direct hydrocarbon electrochemical oxidation in novel low-temperature solid oxide fuel cells. For instance, power densities up to 0.37 W/cm2 were measured for single cells that were operated at 650xc2x0 C. with atmospheric-pressure air as the oxidant and pure methane as the fuel. The measured power densities are competitive with fuel cells operated on hydrogen. As discussed more fully below, such results can be obtained at low operating temperatures (Tc less than 800xc2x0 C.) and/or by incorporating ceria in the anodes of such cells.
In part, the present invention is a method of using a solid oxide fuel cell for direct hydrocarbon oxidation. The method includes (1) providing a catalytic metal anode and a ceria material contacting the anode and (2) introducing a hydrocarbon fuel to said cell, said fuel absent carbon dioxide and/or water in an amount sufficient to convert the hydrocarbon fuel to hydrogen under cell operating conditions. As such, the method is absent a hydrocarbon reforming stage.
The anode of the solid oxide fuel cell can be constructed using a metal catalytic for the cracking of hydrocarbons. Such a metal includes but is not limited to Pt, Ru, Pd, Fe, CO and Ni present at weight percentages of the type described elsewhere herein. Various embodiments of the present invention can also include a lanthanum chromite. Various other preferred embodiments, including oxidation of lower molecular weight hydrocarbons, utilize nickel.
In preferred embodiments of the present invention, the ceria material includes a dopant. Such dopants include but are not limited to various oxides of yttrium, gadolinium and samarium. Highly preferred embodiments include a yttria-doped ceria, having the stoichiometric relationship of (Y2O3)xc3x97(CeO2)1xe2x88x92x, where xe2x80x9cxxe2x80x9d can be about 0.1 to about 0.25. One such embodiment is (Y2O3)0.15(CeO2)0.85, although other such stoichiometries would be known, to those skilled in the art made aware of this invention, to provide a similar or comparable functional result.
With reference to use of a nickel metal and only by way of example, the catalytic anode can comprise a nickel composite. Such a composite can further include ceria and/or zirconia materials or layers of such materials used in conjunction with the nickel metal. Zirconia can be introduced to such a composite as an electrolyte adjacent to and/or contacting the anode. In preferred zirconia embodiments, various dopants can also be utilized, such dopants including but not limited to calcium, scandium, and yttrium. As would be well-known to those skilled in the art and made aware of this invention, other electrolytes can be used, including ceria, strontium-doped lanthanum gallium magnesium oxide, any of which can be doped as discussed elsewhere herein.
The method of the present invention provides for direct oxidation of hydrocarbon fuels, substantially without any reformation reaction. Fuels especially suitable for use herein include, without limitation, C1-C8 alkanes, and the corresponding alcohols. Likewise, combinations of such hydrocarbons can be utilized with good effect, some mixtures for the purpose of approximating natural gas compositions.
In part, the present invention is also a method of using a ceria material to increase hydrocarbon oxidation rates in a solid oxide fuel cell. The inventive method includes (1) providing a solid oxide fuel cell having an anode composite of a catalytic metal and a ceria layer, (2) operating the cell at a temperature less than about 800xc2x0 C., (3) introducing a hydrocarbon fuel directly to the anode and (4) sorbing oxygen with the ceria layer for transfer to the anode for hydrocarbon oxidation. Solid oxide fuel cells can be constructed and/or fabricated using methods and techniques well-known to those skilled in the art, together with use of the cell components otherwise as described more fully herein. In preferred embodiments, the hydrocarbon is methane, ethane or a combination thereof, although other fuels can include those previously discussed. Irrespective of the choice of hydrocarbon fuel, preferred embodiments of such a method include operating the cell, together with its anode, at a temperature between about 500xc2x0 C. and about 700xc2x0 C.
In part, the present invention is also a method for suppressing and/or eliminating carbon deposition during electrochemical oxidation of a hydrocarbon in a fuel cell. The method includes (1) providing a solid oxide fuel cell anode composite of a catalytic metal and a ceria layer, (2) operating the cell at a temperature less than about 800xc2x0 C., (3) introducing the hydrocarbon directly to the anode and (4) oxidizing the hydrocarbon at the anode substantially without carbon deposition on the anode. As with other aspects of the present invention, this method can be effected using fuel cells of the prior art and/or as constructed and/or fabricated as elsewhere described herein. In particular, but without limitation, the anode comprises a catalytic metal selected from the group consisting of Pt, Ru, Pd, Fe, CO and Ni. Regardless, preferred embodiments include introducing oxygen electrochemically at the anode at a rate and pressure sufficient to react the oxygen with any elemental carbon present, whereby carbon monoxide disproportionation and/or hydrocarbon pyrolysis are inhibited. While operating pressures less than 800xc2x0 C. provide the desired effect, such embodiments can be employed beneficially at lower temperatures, typically between about 500xc2x0 C. and about 700xc2x0 C., depending on the anode material and/or the hydrocarbon oxidized.
In part, the present invention is also an anode for direct hydrocarbon oxidation in a solid oxide fuel cell. The anode includes (1) a composite having a catalytic metal and a ceria material (2) such that the metal is present in an amount less than 60 weight percent of the anode. Catalytic metals of the present invention include those known to those skilled in the art as useful for the cracking and/or oxidation of hydrocarbons. In preferred embodiments, such a metal can include those of the type described more fully above. Regardless, the ceria material can be used with or without a dopant. In any event, the anode of this invention is substantially without carbon deposits under cell operating conditions. At lower metal levels, the resent invention contemplates use of a current collector as needed to supplement conductivity.
High power density SOFCs and related methods of this invention operate by direct electrochemical hydrocarbon oxidation without carbon deposition. The anodes described herein provide for rapid hydrocarbon electrochemical oxidation rates. The results, confirmed with a simple thermodynamic analysis, show that SOFC stacks can be operated in the temperature range from ≈500 to 700xc2x0-800xc2x0 C. without carbon deposition. Direct oxidation provides a desirable method for utilizing a variety of hydrocarbon fuels, avoiding the difficulties associated with reforming. Indeed, this may be the only feasible approach for low-temperature SOFCs, since extrapolation of internal reforming rate data below 750xc2x0 C. suggests that reforming rates become prohibitively small.
In part, the present invention is a solid oxide fuel composite, including: (1) a substantially planar, electrically-insulating substrate; (2) a plurality of cathode components on the substrate, each cathode component spaced one from another; (3) an electrolyte on and between each cathode; (4) a plurality of anode components, each anode spaced one from another and corresponding in number to the plurality of cathode components; and (5) an interconnect component contacting the portion of each cathode component and a portion of each corresponding anode component. Such a configuration provides for fuel and oxidant cavities as shown, for instance, in FIG. 7B. Alternatively, a plurality of anode components can be deposited on a substrate, with a corresponding number of cathode components interconnected therewith.
In preferred embodiments, the solid oxide fuel cell composite includes a catalytic metal anode and a ceria material contacting the anode, as described more fully above, for direct hydrocarbon oxidation. In such embodiments, the catalytic metal includes, but is not limited to, Pt, Ru, Pv, Fe, CO and Ni present at weight percentages of the type described elsewhere herein. Other embodiments, preferred or otherwise, can be utilized with comparable effect depending upon the type of fuel. Regardless, preferred embodiments of such fuel cell composites include a doped ceria material. Highly preferred embodiments include a yttria-doped ceria having a stoichiometric relationship such as that provided elsewhere herein.
In part, the present invention can also include a solid oxide fuel cell assembly, including: (1) a substantially planar array of fuel cells on a substrate, each cell having cathode, anode, electrolyte and interconnect component structures, with each component structure of each cell having a sub-planar arrangement of one to another; (2) an oxidant cavity adjacent the substrate; and (3) a fuel cavity adjacent the sub-planar anode arrangement. As discussed above, and as further described in example 14, an assembly configured with anodes on a substrate will provide a converse cavity placement; i.e., a fuel cavity adjacent the substrate and an oxidant cavity adjacent the anodes. Fuels and oxidants useful with such cells and related assemblies are as described herein or otherwise known in the art. Likewise, the requisite cavities and supporting cellular/assembled structures will be understood upon consideration of various aspects of this invention.
A preferred embodiment of such an assembly is illustrated in several of the following figures. In particular, a plurality of such planar arrays, configured with the corresponding oxidant and fuel cavities can provide a stacking configuration such as that portrayed in FIG. 7B.
In part, the present invention is also a method of constructing a series of solid oxide fuel cells, such cells as can be used in conjunction with the composites and/or assemblies described above. Such a method, without limitation, includes one or more of the following constructions: (1) providing a substrate with masks aligned thereon in a predetermined pattern; (2) placing/depositing a first electrode material on the substrate; (3) re-aligning the masks on the first electrode materials, one mask on each such first electrode material; (4) placing/depositing an electrolyte material on the first electrode material; (5) removing the masks and placing/depositing an interconnect material on the first electrode material; and (6) re-aligning the masks on the electrolyte and placing/depositing a second electrode material on the electrolyte.
Such electrode/electrolyte and/or interconnect components can be prepared and integrated on a substrate and with one another as described herein, using thin-film/layer techniques of the prior art, such techniques and straight-forward modifications thereof as would be understood by those skilled in the art made aware of this invention. Successive masking, deposition, and unmasking procedures can be employed to deposit/print cathode, electrolyte, interconnect and anode components on a suitable insulating substrate. Such procedures or fabrication steps would also be known to those skilled in the art, modified as necessary to accommodate use of the component materials described herein or to otherwise achieve the functional and/or performance characteristics desired. Fabrication in this manner on a suitable substrate, provides a planar composite, array and/or assembly of solid oxide fuel cells wherein the cells are integrated one with another. (See, for example, FIG. 7A.) The cellular stacking geometries of this invention have, therefore, the capacity to be two- or three-dimensional. Such procedures can be viewed as analogous to various thin-film/layer techniques used in the fabrication of micro- and nano-dimension integrated devices, hence the reference to integration and integrated stacks.
Individual planar, integrated assemblies can be mounted one above another and between structural components described elsewhere herein and as necessary to provide a functional fuel cell. Such end-plates/caps, fuel feed tubes and associated non-conductive seals are of well-known material choice and construction, the design of which can be as shown in FIG. 7B or, in accordance with this invention, as necessary to provide the desired performance property or parameter.
As discussed above, SOFCs are a very promising energy conversion technology for utilization of fossil fuels and hydrocarbons produced therefrom. The present invention introduces a novel stacking geometry devised to enhance the benefits available from this technology. The geometry involved has all active SOFC components and the interconnect deposited as thin layers on an electrically insulating support. This configuration allows the choice of a support material that provides optimal mechanical toughness and thermal shock resistance. The supports can be in the form of flattened tubes, providing relatively high strength, high packing densities, and minimizing the number of seals required. The integration of SOFCs and interconnects on the same support provides several other advantages including the reduction of electrical resistances associated with pressure contacts between the cells and interconnects, relaxation of fabrication tolerances required for pressure contacts, reduction of ohmic losses, and reduction of interconnect conductivity requirements. The materials used in the integrated stacks of this invention can be similar to or the same used with conventional SOFCs, and long-term stable operation will be achievable. Use of thin layer cell-active components helps to lower overall material costs.
Without limitation as to the scope of this invention and irrespective of the fuel (hydrogen or hydrocarbon) used, the following provides several advantages, attributes and/or aspects pertaining to one or more embodiments of the stacking configurations, geometries and/or assemblies described herein.
1. The support is not an electrically active part of the stack; it can be designed chosen for optimal mechanical properties.
2. There are no separate interconnect pieces, and a reduction in the number of seals is possible. The cells can be fabricated in the form of flattened tubes, such that a seal-less design similar to tubular stacks can be implemented while retaining the high power-to-volume ratios of planar stacks.
3. Because the SOFC components and interconnects are in intimate contact, electrical losses related to pressure contacts are greatly reduced, improving stack performance.
4. Because no separate interconnect pieces and fewer gas-flow channels are required, there is a reduction in stack volume and weight.
5. Large integrated stack elements can be made by increasing the number of cells: there is no need to make very large area cells.
6. Considerable flexibility is provided by way of stack design: for example, individual cell sizes can be varied slightly to account for spatial variations in gas composition and temperature.